Pull cables can be used to apply forces remotely. For example, some bicycles provide brake handles and shifting controls that are coupled to brake assemblies and gear shifters located elsewhere on the bicycle. The bicycle rider applies a force to the brake handle or shifting control, and the end of a pull cable attached to the brake handle or shifting control is moved, resulting in a corresponding movement of the other end of the pull cable, located at the brake assembly and/or gear shifter. These pull cables, also known as “Bowden cables” consist of two parts, an inner cable of twisted steel wire, and an outer cable housing.
Pull cables transmit force from one end of the cable to another end of the cable by a combination of tension on the inner cable and compression to the housing. In some installations, the outer cable housing does not run along the full length of the inner cable, but transmits the compressive force to a bicycle frame by means of housing stops, fittings with holes small enough for the cable, but not small enough for the housing to pass through. Some cable stops feature adjusting barrels.
In many installations, the path of the pull cable includes bends or curvatures. Along the curve, there is increased surface friction between the inner cable and the outer cable housing. This surface friction is due to the non-axial forces exerted at the curvature interface. Tensile force applied to the inner cable result in some force against the inside of the outer cable housing at bends or curves in the pull cable. The tensile force tends to work to straighten the inner cable, causing the inner cable to contact the outer cable housing where the pull cable bends. As the curve radius decreases or becomes more acute, a greater component of the tensile force may be applied to the inside of the outer cable housing. In addition, as a pull cable is drawn with more force over a curved section, the friction resistance becomes increasingly larger. Finally, when the curve becomes smaller than a certain radius or surface friction and/or tensile force become too high, the curve will collapse or kink and render the pull cable system less efficient or inoperative.
Recent pull cable systems replace and/or surround the outer cable housing with a plurality of form-parts so joined to one another as to form a continuous axial sheath around the inner cable. However, these recent pull cable systems can fail at certain curvatures. At decreased curve radii and/or at increased tensile forces, the form-parts may become unattached or out of alignment from each other at the curve, thereby allowing the inner cable to bend or even deform, and thus, greatly reduce the effectiveness of the pull cable system.